There is a demand for vehicles that can travel both in the fore and aft direction and in the lateral direction. Such vehicles may be useful for vehicles and robots that are required to be highly maneuverable in a limited space. The main wheel for omni-directional vehicles normally consists of an omni-wheel that can travel both longitudinally and laterally. The main wheel disclosed in WO2008132779A1 (US2010096905A1) comprises a stiff annular member and a plurality of free rollers rotatably fitted on the annular member via a sleeve member like so many beads of a rosary. Each sleeve member is required to have an arcuate bore configured to be fitted on the annular member and a truly cylindrical outer surface so that the free roller may be able to rotate substantially without any friction or play. JP 3820239B discloses different examples of omni-wheels.
Such an omni-wheel, in particular the one disclosed in WO2008132779A1 (US2010096905A1), can be favorably used for a friction drive device and an omni-directional vehicle incorporated with such a friction drive device. In the omni-directional vehicle disclosed in this prior patent publication comprises a pair of drive assemblies individually actuated by electric motors and a main wheel held between the drive assemblies and frictionally driven by the drive assemblies. See for instance. Each drive assembly comprises a drive disk coaxially opposing the drive disk of the other drive assembly and rotatably supported by a frame, and a plurality of drive rollers obliquely arranged along the circumference of the drive disk at a regular interval so as to be individually rotatable. The main wheel comprises a ring-shaped annular member rotatably supporting a plurality of driven rollers (free rollers) arranged along the circumferential length of the annular member so as to be rotatable around the respective tangential lines of the annular member. The driven rollers are interposed between the two groups of the drive rollers rotatably supported by the two respective drive disks. As the drive disks are turned by the electric motors, the driven rollers are frictionally driven by the drive rollers. When the driven rollers are turned around the tangential directions of the annular member (or around the respective center lines of the driven rollers) by the drive rollers, the vehicle is driven in a lateral direction. When the main wheel is turned around the central axial line thereof, the vehicle is driven in a fore and aft direction. The direction of motion of the vehicle can be selected as desired by suitably adjusting the difference between the rotational speeds of the two drive disks.
According to this previous proposal, it was found that considerable efforts and costs are required to construct the main wheel (traction wheel) of this kind. In particular, each sleeve member must be secured to the annular member while providing a truly cylindrical outer surface. Also, the annular member requires a relatively high cost for fabrication.
Furthermore, the conventional arrangement may not be able to provide an optimum performance. If the annular member is highly stiff, only the lowermost free roller engages the road surface so that the maximum lateral traction is limited. If the annular member is elastic, it is possible to cause two or more of the free rollers to engage the road surface at the same time. However, if the annular member is not stiff enough, the adjoining free rollers may be axially aligned with each other (in parallel with the road surface), and this prevents a favorable engagement between the drive rollers and the free rollers. Also, if the stiffness of the annular member is too low, the adjoining free rollers may engage each other or interfere with each other so that the performance of the main wheel may be seriously impaired.